Power transistor, thyrister, rectifier diode etc. have been used as a semiconductor for a power device since before. For example, when a bipolar power transistor is used as a power control device, the state of operation is controlled in response to a current which is provided to the base. Plenty of base current is needed to operate the power transistor in a saturate region. In general more sufficient current than a value which is divided by a current amplification factor of the transistor is provided as a base current. Therefore a circuit for providing a sufficient base current is needed to control a bipolar power transistor.
In structural feature, MOS transistor is controlled in response to a voltage supplied to the gate. Therefore a steady-state current for the gate is not needed and a circuit for providing the sufficient current like a bipolar power transistor is not also needed.
As mentioned above, no gate current at steady-state is needed to keep the state of the operation because MOS power transistor is a device controlled by voltage supplied to the gate. However a transient current flows into the gate electrode when the state of operation is changed. This gate current is caused by a capacitance between gate an channel. The current flows as a charge current when high voltage is given to the gate electrode and as a discharge current when low voltage is given to the gate electrode.
More sufficient gate current is required for bigger power transistor because large channel is required to flow sufficient current between drain and source so that a capacitance between gate and channel increases.
Therefore large current is transiently required to charge sufficiently the capacitance between gate and channel, and turn on the transistor. If sufficient current is not provided, switching speed is delayed.
When a bias circuit for always supplying sufficient gate current is provided to prevent from a delay of a switching speed of transistor, it has a trouble that a consumption power in the bias circuit increases.